Composite or laminate structures are the basis for many applications in the electronics industry. Advances in printed wiring board laminates have lead to faster, smaller, lighter and cost effective electronic components for use in applications such as radar, antennas, telephony, computer board components, wireless and cellular technology, RF interconnects, and microwave devices. The characteristics of the materials used to make the composites affect the technical abilities and applications for which the composite or laminate structure can be used.
A variety of composite structures are used in the electronics industry. Technical requirements for such composites include the structural integrity of the finished structure, the ability of the individual components to withstand the rigors of assembly, the ability of the assembled structure to withstand a variety of processing conditions, such as those used in making printed wired circuit board (including, e.g., the ability to withstand high temperature conditions as experienced during soldering operations and the ability to interconnect layers by means of plating through vias), the performance properties of the components used and the finished structure (including the dielectric constant, resistance to environmental conditions such as moisture, atmosphere, harsh chemicals, and heat), costs of the components, and costs associated with the manufacture of the finished article.
One component of a laminate is the dielectric material that is used. A dielectric material is an insulating material that does not conduct electrons easily and thus has the ability to store electrical energy when a potential difference exists across it. The stored energy is known as an electric potential or an electrostatic field which holds electrons. The electrons are discharged when the buildup of electrons is sufficiently large. Common dielectric materials include glass, mica, mineral oil, paper, paraffin, polystyrene, plastics, phenolics, epoxies, aramids, and porcelain. The characteristics of the dielectric are determined by the material from which it is made and its thickness.
In electronic circuits, dielectric materials may be employed in capacitors and as circuit board substrates. Conventionally, dielectric constant materials are used in radar or microwave applications and also for circuit miniaturization as the speed of propagation of signal at a constant frequency is proportional to the inverse of the square root of the dielectric constant of the medium through which it passes. Low dielectric constant materials are used for high speed, low loss transmission of signals as such materials allow faster signal propagation, and less space is required in circuitry design or in conductive layers. Low dielectric materials also have radar and microwave applications. If the combination of materials is such that the loss tangent for a material of a given frequency signal is very low, the circuit board will allow very efficient transmission or splitting of the signal without electrical loss related to the hysteresis loop. If a whole circuit were built on low dielectric material, one could amplify the signal only a certain amount at each mounted transistor, and because of the lower power involved, the assembly would reduce the build up of excessive heat and temperature. Consequently, the amplification would be spread over a large space. If all of the dielectric material had a high dielectric constant, there would be more loss at signal splits so that more transistors would be necessary to maintain a specific signal to noise ratio, and more power would be required to operate these components.
One of the common materials used in the production of printed circuit boards, which are used in antennas and other elements of cellular and wireless technology, is glass fiber and/or woven glass materials that are coated with PTFE (polytetrafluoroethylene), cyanate ester, Aramids, and/or PTFE films. These materials have been used because they can be manufactured readily. However, they are more expensive than many other higher dielectric printed circuit materials, and require multiple steps to manufacture. They are also relatively heavy due to its density of about 2.5 gm/cm3. Furthermore, these materials generally have a dielectric constant no lower than about 2.17.
Efforts have been made to provide materials that are lighter and have lower dielectric constants. Such efforts include making a structure in which a microballoon-filled adhesive is used to bond metal foil directly to a rigid polyisocyanurate foam. While potentially useful in manufacturing individual antennas, the method is limited in that there is no true barrier to attack of the foam surface by process chemistries (both aqueous and organic) typical of printed wiring board manufacturing processes once the copper has been etched away. This results in degradation of and/or inconsistency in electrical properties and performance. Another known weakness with the polyisocyanurate foam is degradation when exposed to ultraviolet rays. Thus, this method cannot be used in the high volume continuous manufacturing necessary to produce a product economically.
Other problems also arise during the manufacturing process. For example, scientists have attempted to resolve the issue of degraded electrical performance by using a polyurethane film adhesive to bond copper foil directly to a rigid Baltek polystyrene foam core at 350° F. However, such treatment led to the partial structural collapse of the foam and did not result in an impermeable barrier between the copper and the foam. The resultant product had pinholes in the film/bonding layer, which resulted in the penetration of etch chemicals during processing. Another attempt was to coat the foam itself with a ceramic-filled resin system known to have good electrical properties. Again the foam collapsed due to heat and pressure, resulting in a material that was too dense and the seal between the copper and the foam was still inadequate to eliminate etchant penetration and entrapment in the foam structure. Other composites also have been investigated, such as polyethylene in closed and open cell forms. The results indicate that the material structure and integrity of the product was compromised in these studies. Many of these polyethylene and polystyrene foam materials also cannot survive processing required to plate connecting holes.
There is a need for a dielectric material that has at least one of the desirable characteristics, such as, a low dielectric constant, a low loss tangent, the ability to withstand a wide range of temperatures, the ability to operate in wide range of atmospheric conditions and pressures, and capable of being used in the manufacture of composite structures that can be used alone or in combination with other materials. Such completed assemblies could form electronic components used in electronic devices.